Stress remains inside synthetic resin parts welded by a welding mechanism such as a hot plate welding method, a vibration welding method, or the like, due to heat applied to the synthetic resin parts at the time of welding. It is therefore, necessary to keep the synthetic resin parts at a predetermined temperature for a time not shorter than a predetermined time, to thereby eliminate the residual stress.
Thus, there exists a prior art technique in which welded parts are carried on a predetermined course in a substantially closed annealing furnace while being heated at a predetermined temperature for a predetermined time in order to eliminate residual stress.
For example, a prior art steam annealing furnace for annealing welded parts with hot air is known. However, the problem with this prior art device is that the heating efficiency was insufficient, so that it took too much time to anneal.
Thus, an annealing furnace using infrared lamps having a superior efficiency of heating is used in the prior art. FIG. 2 is a schematically plan view showing the outline of such an annealing furnace a. Welded parts b, b, . . . are introduced into the annealing furnace a through a doorway c, and mounted on trays d, d, . . . , respectively, and moved in the direction of the arrow by a not-shown carriage means. While the welded parts mounted on the trays d, d, . . . make a round in the annealing furnace a, the welded parts are heated by not-shown infrared lamps, kept at a predetermined temperature for a time not shorter than a predetermined time to be annealed, and then extracted from the doorway c.
However, in the above-mentioned prior art annealing apparatus, there was a problem that welded parts b close to the infrared lamp rose too high in temperature to thermally deform when the doorway c was left closed for a long time for some reasons, for example, because a worker left his/her position.
In addition, in the above-mentioned annealing apparatus, there was a problem that a large plane space was required for installation of the annealing furnace a because the trays d, d, . . . moved in plane in the annealing furnace a.
Taking the foregoing circumstances into consideration, it is therefore an object of the present invention to prevent a temperature from rising locally in a closed space so as to make the temperature in an annealing furnace uniform, to thereby improve the annealing efficiency.